As is known, loads in various directions act on wind turbine blades used in a wind turbine generator system (hereinafter referred to as a windmill). In particular, at the time of a strong wind, a heavy load acts on a blade root portion of each wind turbine blade in a flap direction. On the other hand, windmills are generally required to have a prolonged service life. Thus, there have been growing needs to provide wind turbine blades with additional reinforcements as needed in order to deal with possible degradation of or possible fatigue damage to the wind turbine blades resulting from long-term use thereof.
In regard to reinforcement of the wind turbine blades, Patent Document 1 discloses a technique for adding reinforcing ribs to an inner circumference or an outer circumference of the blade root portion of the wind turbine blade along a circumferential direction of the blade root portion. Specifically, in the technique disclosed in Patent Document 1, circular-arc-shaped reinforcing ribs are additionally provided around a part or all of the inner or outer circumference of the blade root portion. Both ends of the circular arc of each of the ribs are tapered along the circumferential direction of the blade root portion.